Most job posts seem to require the "alphabet soup" you can get from job hopping. E.g. 3-5 years in Java, 3-5 Years in C++, 3-5 Years in .NET. Often a job like that needs someone who hopped 3 jobs because often you use just one in a job. Also most jobs I have had tend to repeat themselves over and over again after the first year or two. I am a quicker learner than a lot so it may not be as bad. But the first 6 months or so you pick up the day to day responsibilities of the job and probably any new nuances of the technology. For my first job it was more like a year. After that it is just repeating the same 6 months/1 year of experience over and over again doing the same thing. People who I know who hopped more got more varied experience. Some of them hop every 2 years or every 1 year. They get Ruby/Java/C/PHP/etc. and can jump on jobs with a variety of requirements. Meanwhile I change less often so I don't have such a variety of experience which makes it harder to change. Generally I don't even get an interview until an employer gets desperate.... And if they find a competent job hopper they won't even get desperate enough to call me in...

Also from a salary perspective you really suffer if you stay at the same place too long. I don't know why but when you work for a company year after year, your "loyalty" is often "rewarded" with a token 2-3% raise that barely keeps up with inflation. If you job hop usually you get 10-20% just for the hop. If the market has really picked up you could end up with way more. It's not all about the money for the sake of money, but future employers judge you based on your salary. If you stayed at the same job 5 years and then switch they will wonder why your salary is so low and be hesitant about hiring you....

Overall I think the situation is different with a consulting company where you can work for the same company but work on a variety of projects. Also some companies have strong internal mobility programs to help workers get around. But others are too small to have one or it is a joke. But on a per job basis I'd say you learn more by changing especially in a big company where the roles are more specialized. Although you don't always need to change companies for a new job.